The present invention relates generally to optical fiber waveguide fabrication. More particularly, it concerns a method and apparatus for the fabrication of optical fiber preforms using a plasma forming torch which results in improved efficiency as compared with prior systems.
Optical fiber waveguides typically include a light transmitting core surrounded by a cladding. The optical fiber is manufactured from a high-purity silica in which the index of refraction of either the cladding or the core is adjusted during manufacture to provide a lower index of refraction in the cladding than the core. Various processes have been developed for manufacturing optical fibers, many of which use a silica starting preform having the same cross sectional geometry as the finished fiber and a diameter dimension (e.g., 25 mm) that is several orders of magnitude greater than that of the finished fiber. One method of fabricating a preform involves bringing together silica precursor gases, such as silicon tetrachloride and oxygen, and heating these gases using a hydrogen fuel gas in a temperature range between 1400.degree. and 1800.degree. C. The precursor gases undergo a reaction to form silica particles which are deposited onto a suitable substrate or otherwise accumulated. Certain dopant materials can be added to the precursor reaction gases to alter the index of refraction of the resulting silica. For example, boron or fluorine compounds can be added to effect a lowering of the index of refraction while compounds of phosphorous or germanium can be added to effect an increase in the index of refraction. After sufficient material is deposited, the mass of deposited material can be consolidated by heating to form a clear rod that can be subjected to various physical shaping steps and then drawn in one or more drawing steps into a finished fiber.
One drawback associated with the described process is that the hydrogen fuel gas can combine with available oxygen to form hydroxyl ions (OH) in the resulting preform and the finished optical fiber. The presence of these OH ions changes the spectral absorption of the silica so that light of selected wavelengths is undesirably attenuated in the finished fiber.
In an effort to reduce the formation of undesirable OH ions consequent to the use of hydrogen fuel in the above-described silica forming reaction, an electrically generated plasma has been used as the heat source for the reaction so that the silicon tetrachloride, oxygen, and dopant or dopants undergo reaction in the heat provided by the hydrogen-free plasma. Since hydrogen is not used as the fuel gas, the problem of OH formation in the resultant preform is greatly reduced. Since plasmas generate high temperatures, on the order of 10.degree.-30,000.degree. C., care must be exercised in introducing the precursor materials into that portion of the plasma or in a region adjacent to the plasma where the temperature is best suited for the formation of the doped silica particles.
The use of a plasma as the heat source in the silica-forming reaction reduces the problem associated with the formation of the OH ions. As can be appreciated, it would be desirable to fully utilize the energy required to form the plasma in the production of doped optical material.